More than 8 percent of patients taking Sovaldi (sofosbuvir), the latest hepatitis C virus drug from Gilead Sciences, do not complete the full 12-week course of treatment, according to a CVS Health Research Institute study reported by Forbes.com.

Pharmacy data collected during the past nine months shows the dropout rate has been roughly four times that observed in clinical trials.

Whether these patients will have to re-start their treatment to cure the virus remains unknown.

However, it is estimated these patients have already incurred health care costs between $28,000 and $56,000 each without knowing if they were cured. The $1,000-per-pill hep C drug boasts up to 90 percent efficacy when taken as directed.

Meanwhile, Gilead’s upcoming fixed-dose combination pill of Sovaldi with a drug called ledipasvir is only an eight-week course of treatment. The U.S. Food and Drug Administration will announce an approval decision October 10.